1. Field of the Invention
The invention relates to improvement in stitching accuracy of the wide-angle image generated by multi-camera system.
2. Related Art
As shown in the prior art of generating a wide-angle image by multi-camera system as claimed in US Patent No. US2003/0235344 A1 and US2005/0025313 A1, a system for generating a seamless wide-angle image using image stitching technique has been disclosed. However, the stitching problem caused by lens distortion was not addressed. The system for generating a wide-angle image utilizes the features in the image overlapping region for stitching point searching and calculation of the image transformation parameters. Due to the lens distortion, the features in the overlapping region are distorted and fail to match perfectly with the adjacent image. The mismatches in the stitched wide-angle image are visible and cannot be easily corrected.
Referring to FIG. 1 that shows a conventional multi-camera system 100, the multi-camera system 100 includes a camera array 101 and an image stitching engine 102. The image stitching engine 102 acquires multiple images from the camera array 101. The images are then transformed onto a common plane and blended, thereby generating a seamless wide-angle image. Each camera of the camera array 101 is connected to an aperture controller 103 by a control bus. An exposure control signal is sent by the image stitching engine 102 to the aperture controllers 103 based on the brightness of the acquired images for feedback control.
Images acquired from the camera array usually overlap. The overlapping regions are used for blending, stitching point searching and calculation of the image transformation parameters. Generally, the conventional multi-camera system utilizes block matching for stitching point searching in the overlapping regions.
Either barrel or pin-cushion distortion happens to the lens of cameras. The distortion brings two major problems to image stitching, i.e., inaccuracy of stitching point searching and inaccuracy of image transformation. FIG. 2 illustrates an example of lens distortion, where the distorted features in the overlapping region introduce deviation on the searched stitching points when block matching for stitching point searching is applied.
When the cameras of the camera array 101 shown in FIG. 1 are rotated, the images captured by the camera array 101 need to be transformed onto a reference plane for stitching. The transformation matrix is calculated by giving four coordinates on the reference plane and four coordinates on the rotated plane which is to be transformed.
Referring to FIG. 3 that shows four points deviated from the ideal coordinates due to lens distortion, this deviation causes inaccuracy of the calculated transformation matrix obtained from the distorted points. As a result, the rotated plane is not transformed exactly to the reference plane, and the features in the overlapping region of the transformed image do no match with those of the reference image. It is very difficult to compensate the mismatches by blending.
In view of the above, the present invention provides a method for correcting lens distortion of the system and a device for implementing the same, so as to overcome the mismatches of the features in the overlapping region of the multiple images and improve the accuracy of wide-angle image stitching, thereby generating a seamless wide-angle image.